In many cases, a hydrogen-containing gas is used as a fuel gas when a fuel cell power generating system generates electric power. Generally, a fuel processing apparatus configured to generate the hydrogen-containing gas by utilizing steam reforming is attached to the fuel cell power generating system.
The fuel processing apparatus includes a reformer configured to generate the hydrogen-containing gas by a steam-reforming reaction and a heater configured to supply heat necessary for the steam-reforming reaction. A reforming catalyst, such as a precious metal based catalyst or a Ni based catalyst, is used in the reformer. Examples of the precious metal based catalyst are platinum, ruthenium, and rhodium. A hydrocarbon based raw material, such as a natural gas, LPG naphtha, gasoline, or kerosene, or an alcohol based raw material, such as methanol, and moisture are supplied to the reformer. Then, the reformer is heated by the heater to a temperature suitable for the steam-reforming reaction, and the steam-reforming reaction occurs at the reforming catalyst. Thus, the reformer generates the hydrogen-containing gas.
In this steam-reforming reaction, carbon monoxide (hereinafter referred to as “CO”) that is about 10 to 15% (dry gas base) of the hydrogen-containing gas is generated as a subcomponent. The CO poisons a catalyst used in an electrode of a fuel cell to deteriorate an electric power generating ability. Therefore, in order to reduce a CO concentration of the hydrogen-containing gas, a shift converter and a CO remover are attached to the fuel processing apparatus. For example, a precious metal based catalyst, a Cu—Zn based catalyst, or a Fe—Cr based catalyst is used in the shift converter. Examples of the precious metal based catalyst are platinum, ruthenium, and rhodium. The shift converter is controlled to a temperature (shift reaction temperature) suitable for a shift reaction and causes the shift reaction between the CO and steam to generate hydrogen and carbon dioxide. In many cases, the CO concentration of a reformed gas is reduced to about 0.5% or lower. The CO remover carries out a CO oxidation reaction by using air supplied thereto. With this, the CO concentration of the reformed gas is reduced to preferably 10 ppm or lower. The precious metal based catalyst, such as platinum, ruthenium, or rhodium, is used as the catalyst.
In order to improve an energy efficiency by using the fuel cell power generating system at home, it is desirable to start up or stop the fuel cell power generating system in accordance with home electric power load and heat load. Here, the reformer is operated at about 650° C. Therefore, when the reformer stops, the temperature thereof decreases, and the internal pressure thereof decreases. When the internal pressure of the reformer decreases to a negative pressure with respect to an atmospheric pressure, the air may get into the reformer from outside. As a result, in the case of using the Cu—Zn based catalyst as the shift catalyst, the air may get into the reformer, and this decreases an activity. Therefore, a method for preventing the air from getting into the reformer while the reformer stops is required.
Here, proposed is a method for preventing the air from getting into the reformer by supplying the raw material to the reformer to maintain the internal pressure of the reformer when the internal pressure decreases (see PTL 1, for example).